Typically, the superconducting rotating machine has a stator and a rotor, electromagnetically connected with each other. The rotor comprises a multipole rotor core and one or more field coils mounted on the rotor core. The rotor core is made of a magnetic material such as iron, with a longitudinal axis extending along a rotational axis of the rotating machine (see, for example, JP 2003-125573 A).
In order to maintain the superconductivity of the superconducting coils on the rotor core, the superconducting coils are cooled down to approximately 30K, for example, through a conduction cooling. Conventionally, various techniques have been known for cooling the superconducting coils of the rotor core, such as one in which the superconducting coils are immersed in a cryogen such as liquid helium or liquid nitrogen, or another in which the superconducting coils are brought into contact with a cold head of the cooling machine so that cooling power generated in the cold head is transferred to the superconducting coils (see, for example, JP 6-302869 A, JP 10-9696 A, and JP 2003-151822 A).
The rotor core disclosed in JP 2003-125573 A is made of a heavy solid member and therefore has a high heat capacity, which needs a long time for a uniform cooling of the rotor core in its entirety down to a certain temperature. Also, an incorporation of the superconducting coils into the electromagnetic device may cause a very high magnetic field to result in a saturation of a magnetic field of the magnetic member if it is made of iron. This prevents such material from being used for the magnetic member. Further, as described above, the superconducting coils should be cooled down to approximately 30K, for example, in order to maintain the superconductivity of the superconducting coils. In the meantime, the carbon steel including iron can result in cold brittleness at the extremely low temperature of 30K. To prevent this, the rotor core should be made of a material with high resistance against the cold brittleness.
According to the cooling methods disclosed in JP 6-302869 A, JP 10-9696 A, and JP 2003-151822 A in which the superconducting coils are immersed in the cryogen such as liquid helium or liquid nitrogen, the extremely cooled medium is transported into a container for receiving and cooling the superconducting coils, which requires various facilities and special cares for handling the cooled medium and, therefore, results in various disadvantages in spaces and costs for facility installations and in maintenance. In addition, it is relatively difficult to control the cooling temperature.
According to another cooling method in which the cold head of the cooling machine is made into contact with the superconducting coils for its cooling, the cooling machine is mounted on the rotating rotor core, which results in that the cooling machine is rotated with the rotor core. This needs additional weight and space for rotating the cooling machine. Also, a continuous supply of the pressurized helium gas needs that a rotating seal mechanism is mounted on the rotational cooling machine. Actually, however, a rotating seal mechanism with an extremely high sealing property is not available at the moment and, therefore, this method seems to be impractical.